CRAWFORDSVILLE, Indiana (Reuters) - Senator Richard Lugar of Indiana is one of the Senate's conservative titans, the courtly, silver-haired elder of the state's Republican Party who has been unopposed in primary elections for 35 years.
So why is the Senate's longest serving Republican now shaking hands outside factories and traipsing across Indiana fueled by McDonald's milkshakes, his favorite indulgence?
The short answer was plastered on bumper stickers outside a recent gathering of Indiana Tea Party groups: "Retire Lugar," they said. The long answer is that the political world around Lugar has changed.
For many years Lugar was seen as a distinguished, rock-solid Republican senator from a heartland state with a strong foreign policy background and a record of bipartisanship, especially on the complex international issue of nuclear proliferation.
But in post-Tea Party America, words like "compromise" and "bipartisanship" that were once considered virtues are now reviled as vices by a conservative insurgency intent on taking over the Republican Party and moving it further to the right.
"We are grateful for Lugar's long years of service to Indiana," explained Greg Fettig, a landscaper who heads Hoosiers for a Conservative Senate, a Tea Party umbrella group leading the charge against Lugar. "But he has moved too far to the left and has to go."
A collection of groups affiliated with the conservative Tea Party movement have banded behind state treasurer Richard Mourdock in the first Republican primary challenge Lugar has ever faced. Democrats, sensing vulnerability, also are taking him on in his first general election challenge since 2000.
The Lugar race is seen nationally as an important test of the viability of the Tea Party, which some believe has lost momentum since a populist surge in 2010.
With $4 million in the bank, Lugar holds a 10-to-1 fundraising advantage over his Republican challenger. But the Tea Party groups hope to overcome the financial disadvantage by using 5,000 volunteers to muster a massive get-out-the-vote campaign.
Mourdock spokesman Chris Conner said they will focus on Lugar's record and will raise enough funds to last until the May 8 primary.
"On all of the important issues... Senator Lugar has come down on the wrong side," Conner said.
Lugar, who turns 80 in April, has cast a number of votes that have raised the ire of his most conservative constituents.
Tea Party activists condemned his votes to raise the U.S. debt limit, to support the 2008 bank bailout and to confirm President Barack Obama's Supreme Court appointees Sonia Sotomayor and Elena Kagan - votes for which Lugar makes no apology.
The conservative Club for Growth, which has endorsed Mourdock, intends to rate Lugar 80 percent in 2011 on its upcoming congressional scorecard, where any ranking over 90 percent is considered praiseworthy. That brings Lugar's lifetime score to 65 percent.
As recently as 2008 Lugar scored just 54 percent, while his South Carolina colleague Jim DeMint, a Tea Party favorite, scored 100 percent.
"Senator Lugar has clearly moved further to the right recently, but even now his record is not truly conservative." said Club for Growth communications director Barney Keller
Opponents also have focused on the fact that since 1977 Lugar has not maintained a residence in Indiana, but has lived in Virginia. While ruled legal by Indiana's attorney general in 1982, conservative critics portray Lugar as the consummate Washington insider who does not even live in his home state.
"For 35 years rightly or wrongly this (residency) issue has not been raised," Lugar told a group of supporters during a recent campaign stop in Crawfordsville, a town of around 16,000 some 50 miles northwest of Indianapolis. "I'll leave it to your imagination why it has been raised now."
VOTER TURNOUT IS KEY
With the May 8 primary looming, the Tea Party groups are trying to capitalize on their grassroots organization to bring voters to the polls.
Charmian Klem of the group Freedom Makers of Dubois County has been knocking on doors for Mourdock for weeks and recently rented a booth at a local gun show on his behalf.
"I'm always happy to talk about Richard Mourdock," said Klem, 31, an independent representative for cosmetics firm Mary Kay.
Mourdock was the choice of Indiana Tea Party groups at a September convention where they used a straw poll to decide a preference.
John Callahan of the Green Tea Patriots attended the orderly proceedings -- a world away from the rowdy, angry Tea Party rallies of 2009.
"We're far better organized than we used to be and we're working together," he said. "Now we are all on the same page."
On the Democratic side, research by the campaign of U.S. Representative Joe Donnelly, who is challenging Lugar, found poor polling numbers for a sitting senior senator.
"It doesn't look like a straight shot for Lugar," said Indiana Democratic Party spokesman Ben Ray.
Lugar seems to be taking the challenges in stride.
In an interview, Lugar rejected criticism of his votes for Obama's Supreme Court picks, saying he did so "on good faith." He said refusing to raise the debt limit would shut down the U.S. government. "I am not prepared to put up with that kind of destruction," he said.
Campaign director David Willkie said Lugar now has 500 volunteers who've logged 800,000 phone calls to potential voters.
Willkie said Tea Party support for Mourdock was artificial and created by conservative Washington-based groups like FreedomWorks - run by former Republican House Majority leader Dick Armey - which has endorsed Mourdock.
A recent Lugar campaign poll shows the senator with a 25-point lead over Mourdock, compared to a 1-point difference a couple of months ago.
Lugar, whose license plate in Washington is "S-3" to connote his seniority in the chamber, said he remains confident.
"We've been there (in the Senate)," he said. "And we'll continue to be there."
(Additional reporting by Eric Johnson, Editing by Marilyn W. Thompson and Christopher Wilson)